Tag Archives: Planetary science

Various Interesting Articles

Thin section photomicrograph of a gabbro, (crossed polarizing filters).  Image credit: Siim Sepp (CC-BY-SA).
Thin section photomicrograph of a gabbro, (crossed polarizing filters). Image credit: Siim Sepp (CC-BY-SA).

There have been a couple of interesting articles I’ve come across recently, which are worth mentioning.

First, Emily Lakdawalla has an excellent summary of the Pluto discoveries from both the American Geophysical Union’s Fall Meeting and the [NASA] Division of Planetary Science meeting. There’s a lot of new stuff there, and it’s pretty exciting.

Second, the Joides Resolution blog (the Joides Resolution is an ocean sediment coring vessel) has a series of posts (1, 2, 3) on geologic thin sections. Not surprisingly, the thin sections pictured are from rocks such as gabbros or sheeted dikes, which are expected in oceanic crust and in ophiolites (oceanic crust exposed on land). There’s a great exposure of the Coast Range Ophiolite just west of Patterson, CA, in Del Puerto Canyon, which is described in a recent blog post by Garry Hayes.

Third, Dave Petley has a great post on The Landslide Blog about the recent landslide in Shenzhen, China. I find landslides fascinating, and always learn something when I read The Landslide Blog.

Geoscientist’s Toolkit: Fluxgate Magnetometer

Fluxgate magnetometer; coil is around 1 cm in length.  Image credit: Zureks (CC-BY-SA).

The fluxgate magnetometer—not to be confused with the flux capacitor—is a nifty tool for determining the strength and direction of a magnetic field.

It works by using an alternating current to induce an alternating magnetic field in a magnetically permeable core (ferrite core), saturating the core. The magnetic field then induces a current in a secondary winding. My apologies for not having an open-use schematic, but the ones here and here are quite good, plus have a more nuanced explanation.

Absent an external field, the induced current will be equal to the driving current. However, in a magnetic field, one direction will saturate more easily and the other less easily, because the permeable core will be reacting to the external field. As a result, the secondary windings will have a current imbalance when compared to the driving winding, and the imbalance will show up both on the rise and fall of the driving waveform. The imbalance has a frequency of twice the drive frequency. Also, this design detects magnetization in one direction only. For a full 3D characterization of the direction of the magnetic field, it takes three magnetometers, each perpendicular to the others.

One of the early applications of fluxgate magnetometers was the detection of submarines (large metallic bodies). Indeed, through this type of study, the alternating magnetization of rocks along the sea floor of the Atlantic Ocean was discovered, with bands parallel to the Mid-Atlantic Ridge. These data gave strong evidence in support of plate tectonics.

Magnetic field anomalies of the world.  Image credit: J.V. Korhonen,J. Derek Fairhead, M. Hamoudi, K. Hemant, V. Lesur, M. Mandea, S. Maus, M. Purucker, D. Ravat, T. Sazonova & E. Th├ębault, 2007, accessed via SDSU.
Magnetic field anomalies of the world. Image credit: J.V. Korhonen,J. Derek Fairhead, M. Hamoudi, K. Hemant, V. Lesur, M. Mandea, S. Maus, M. Purucker, D. Ravat, T. Sazonova & E. Th├ębault, 2007, accessed via SDSU.

But the magnetometer’s usefulness doesn’t stop there! Earth’s magnetic field extends out into space, where it interacts with magnetic fields from the solar wind. By measuring the magnetic fields, scientists can study the interactions between Earth’s magnetosphere and the solar wind, interactions which can give us auroras.

Aurora in Minnesota.  Image credit:  Charlie Stinchcomb (CC-BY)
Aurora in Minnesota. Image credit: Charlie Stinchcomb (CC-BY)

Perhaps an even more exciting application is the study of magnetic fields near the Moon. NASA’s ARTEMIS mission (using repurposed THEMIS spacecraft) is flying two magnetometers around the Moon. Heidi Fuqua, a scientist at UC Berkeley, and her collaborators are using the magnetic data gathered by the ARTEMIS satellites to study the Moon’s interior. Depending on the size and conductivity of the Moon’s interior, the magnetic field will have differing responses to the induced magnetic field from the solar wind. It’s pretty neat stuff!

Space and Gravity

Solar flare, with Earth for scale.  Image credit: Karl Battams and NASA SDO.
Solar flare, with Earth for scale. Image credit: Karl Battams and NASA SDO.

Space: An Out-of-Gravity Experience opened last week at the Science Museum of Minnesota. I have seen the exhibit, and it is spectacular.

One of the major points the exhibit makes is about gravity, and whether or not there is gravity at the International Space Station, or on the way to Mars or other planets.

Here on Earth, the influence of gravity is pretty obvious. If you throw a ball, it will fall back to Earth fairly quickly. Were you on the Moon or Mars, you could hop around in your space suit, secure in the knowledge that gravity would pull you back down, and insecure about whether your rocket would take you back to Earth.

I will answer that with two questions, which you could imagine asking a stranger in this order:

  1. Why are astronauts are apparently weightless when here on Earth we’re firmly held?
  2. Why, despite the Sun’s great mass, do we not fall into the Sun?

Why don’t we fall into the Sun?! What an absurd question to think about!

The answer has to do with what it means to be in orbit. Randall Munroe of XKCD explains:*

Space is like this:

Imagine you have a machine which can shoot a baseball horizontally at any speed you choose (under the speed of light, and you probably don’t want it getting near that fast anyway). As with many physics thought experiments, we will ignore air resistance. When you start out at reasonable speeds, similar to that which a baseball pitcher throws, it will hit the ground. As you increase the speed, it goes farther and farther before reaching the ground. Keep this in mind.

The Earth is not flat. Although it can seem that way in some areas, the Earth is indeed roughly spherical. At some speed, the baseball will fall toward the Earth in an arc which parallels the Earth’s surface. That speed is the orbital speed (about 8 km/s).** Above about 11 km/s, the baseball would leave the Earth, never to return unless affected by another body. Orbiting the Sun, Earth is moving around 30 km/s along its orbit.

When astronauts experience weightlessness, it is because the spaceship they are in is falling to Earth at the same rate as they are. Think of it like a roller-coaster starting it’s drop, but that just keeps going down and down and down.

Here’s another question for you: which body exerts more gravitational pull on you, the Earth, or the much-more-massive Sun?

To find out, let’s do some (easy!) math. We know from Wikipedia that the attractive force due to gravity is G*m1*m2*d-2, where G is the gravitational constant, m is the mass of the two objects in question (e.g. you and the Earth), and d is the distance between their centers. Approximate a human mass as 60 kg, use your favorite search engine to find the mass of the Earth and the Sun in kg, the radial distance of the Earth in meters, and the orbital distance of the Earth from the Sun in meters, and plug away.

The result? Earth pulls on you more than the Sun, by a factor of ~1600. This makes sense. If the Sun pulled you more strongly than the Earth, you would move toward the Sun, fall in, and die.

Between the Sun and the Earth, there is a point where the gravitational influences and centrifugal force all cancel each other out. This point is the Lagrangian point L1, and is a gravitational sweet spot much like an orbital speed is just right.

Now that things are just right, I should probably wrap things up. Any further addition will send me diving down the rabbit-hole again!

* Incidentally, xkcd and particularly the xkcd what-if comics are among my favorite things to read. I almost think an Ig-Nobel Prize might be deserved for cartoons which make us laugh, then think.

** The orbital speed is somewhat dependent on orbital distance; things further out have a slightly lower orbital speed.